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On January 08 1970, this was accomplished in the evening. The studio used for this overdub was at Olympic Studios in Barnes, England. George also ad-libbed his way through the slide and piano solos and the song ends with George saying "Rhythm and Blues".... The song at this point and on the same evening was mixed by Glyn Johns and overdub became the official vocal.
Later in the year on February 28, and for some unknown reason, Malcolm Davies over at EMI re-mixed the song once again. This was not used.
Finally, a month later Phil Spector came in and re-mixed the song once again in stereo for the "Let It Be" project. After the acoustic guitar intro, the instrument disappears and emphasis is on the piano, John's slide and the drums with the new vocal centered. This final mix by Spector would be used on the Apple LP/CD "Let It Be".